1. Field of the Invention
Embodiments of the invention are generally directed to the field of light field sensing and light field image detection. More particularly, embodiments of the invention are directed to a lens-less, angle-sensitive pixel (ASP) sensor and ASP devices having increased quantum efficiency and pixel density, which can measure the intensity and incident angle of a light field to provide an image of the light field. Embodiments of the invention further include, without limitation, imaging methods associated with said sensor and device embodiments, and applications thereof.
2. Related Art Discussion
Conventional imaging uses a large array of light sensors to create a map of light intensity at an image plane. However, this intensity map fails to capture incident angle, polarization angle, and other properties of light rays passing through the image plane. A complete description of these additional parameters defines the light field or, “flow” of light, at the image plane.
Michael Faraday first proposed the concept of light as a field in the mid 1800's. This concept was expanded by the theory of a “light field” in three-dimensional space. At a given point, the light field is defined by the infinite collection of vectors that represent the light arriving at the point from all angles. The light field can be formally defined by a “plenoptic function” of multiple variables. The plenoptic function parameterizes the light rays passing through all space in terms of intensity, I, which is dependent on position in space (x, y, z), direction (θ, φ), wavelength (λ), time (t), and polarization angle (p). Hence, I(x, y, z, θ, φ, λ, t, p) is the complete representation of a visual scene and contains all possible views of the scene.
Measuring the plenoptic function would require an observer to be able to determine the intensity of every ray, for every wavelength, at all instants in time and at every point in space. Clearly, perfect determination of the plenoptic function for any practical scene is impossible. However, a number of techniques collectively known as light-field imaging have been devised that can record aspects of the plenoptic function beyond simple intensity at a plane. One reported method is to use an array of pinhole cameras, where each camera captures the incident angle-dependent intensity I(θ, φ) at a particular location, (x0, y0). Cameras at different positions (xi, yi) capture a slice of the plenoptic function, I(x, y, θ, φ). Arrays of conventional cameras can also be used, as can camera scanning, or multiple masks. Small-scale solutions have used microlenses to emulate camera arrays. However, all of these approaches require a significant number of parallel or moveable optical components to capture information about the light field beyond a simple intensity map.
Recording information about the light field of a scene provides a more complete description of that scene than a conventional photograph or movie, and is useful for a number of applications. The light field allows prediction of illumination patterns on a surface given known sources and the three-dimensional reconstruction of scenes (e.g., “light-field rendering” or “three-dimensional shape approximation”). FIGS. 1a, 1b show how one aspect of the light field, e.g., incident angle, can be used to localize a light source in three-dimensional space. Capturing the light field also permits construction of images with an arbitrary focal plane and aperture. This capability is useful in both photography and in microscopy for obtaining multiple focal planes without moving optics.
A wide variety of applications require information about the three-dimensional structure of microscale samples. Direct capture of this information using commodity semiconductor chips with no additional hardware would reduce the size and cost of many instruments and assays by orders of magnitude. Existing solid-state image sensors employ large arrays of photosensitive pixels that capture an intensity map of incident light, but no angle information. In typical imaging applications, a lens is required to ensure that the intensity map represents some object of interest. Without a lens, one must rely purely on the information contained in the light rays striking the image sensor. If a sample is placed sufficiently close to the image sensor and illuminated, the resulting intensity map will typically contain some blurred two-dimensional spatial information. Three-dimensional information is completely lost. Information contained in the incident angle of light rays is of interest because it contains further recoverable spatial information.
To date, macroscopic angle-detectors have been demonstrated in unmodified integrated circuit technology. Pixel-scale angle-sensitive structures have been demonstrated on chip but require post-assembled arrays of microlenses, which significantly increase cost and complexity over the manufacture and use of standard imagers.
Another reported technique involves silicon-on-insulator (SOI) structures utilizing regions of metal that are large relative to the wavelength of the light to generate a shadow on an underlying photodiode. This approach has been reportedly used to perform a single angle measurement but is not well suited to deployment in imager arrays.
The inventors recognize that solutions and improvements to the shortcomings and challenges in the prior art are necessary and would be beneficial. More specifically, in contrast to other approaches that require multiple lenses and/or moving parts, devices that are monolithic, require no optical components aside from the sensor itself, and which can be manufactured in a standard planar microfabrication process (e.g., CMOS) would be advantageous in the art. The embodiments of the invention disclosed and claimed herein successfully address these matters and achieve these goals.
The inventors further recognize that the metallic structures used to form the micron-scale fine-pitch transmission amplitude gratings to create the interference patterns from the incident light field, of the instant invention, block a significant fraction of the available light. While reduced light sensitivity is not a significant problem for many applications, maintaining high sensitivity comparable to that of a traditional photodetector permits more widespread deployment of angle-sensitive imagers. In addition, the combination of this ‘top’ grating and an ‘analyzer’ grating as described herein, results in a structure of relatively significant size as well as sub-optimal quantum efficiency (QE). It would also be beneficial to improve angular acuity and reduce the wavelength dependence of the previously embodied ASPs. Accordingly there is a need for an improved ASP apparatus and associated systems and methods that address these problems and concerns without compromising basic function or CMOS manufacturing capability.